Cellular communication systems are currently being developed and improved for Machine Type Communication (MTC), communication characterized by lower demands on data rates than for example mobile broadband, but with higher requirements on, e.g., low-cost device design, better coverage, and ability to operate for years on batteries without charging or replacing the batteries. In the Third Generation Partnership Project (3GPP) Global System for Mobile Communication (GSM)/Enhanced Data Rates for GSM Evolution (EDGE) Radio Access Network (GERAN) specification group, cellular communication systems are being improved and developed in the feasibility study named VODAFONE Group Plc., “GP-140421: New Study Item on Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things,” 3GPP TSG-GERAN Meeting #62, May 26-30, 2014. Both GSM evolution and new “clean slate” systems are being developed. These clean slate solutions are narrowband systems with a carrier bandwidth of 200 kilohertz (kHz) that target improved coverage compared to today's GSM systems, long battery life, and low complexity communication design. One intention with this solution is to deploy the narrowband carrier in spectrum that is currently used for GSM, by reducing the system bandwidth used by GSM and deploying Narrowband (NB) Machine-to-Machine (M2M) communication in the spectrum that becomes available. Another intention is to reuse existing GSM sites for the deployment of NB M2M. Narrowband Long Term Evolution (LTE) is one such solution that borrows most of the design principles from LTE and is currently being proposed as one of the competing proposals for this study. While NB LTE satisfies all the requirements put forward by MTC-type applications, it also features backward compatibility with the current LTE radio access technology, making it very attractive.
In cellular communication systems, devices use a cell search procedure, or synchronization procedure, to understand the cell(s) to which the devices are to connect. Essential functions of a cell search procedure are to detect a suitable cell to camp on and, for that cell, obtain the symbol and frame timing and synchronize to the carrier frequency. When synchronizing to the carrier frequency, the device needs to correct any erroneous frequency offsets that are present and perform symbol timing alignment with the frame structure from the base station. In addition, in the presence of multiple cells, the device also needs to distinguish the particular cell on the basis of a cell identity (ID). Thus, a typical cell search procedure consists of determining the timing alignment, correcting the frequency offset, and obtaining the correct cell ID.
Cell search in LTE uses two special signals, namely, the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). As illustrated in FIG. 1, the PSS and the SSS are in the seventh and sixth Orthogonal Frequency Division Multiplexing (OFDM) symbols of the first and sixth subframes (labeled as subframes 0 and 5) of every 10 millisecond (ms) frame. Note that this design is for Frequency Division Duplexing (FDD) systems only. In systems operating in Time Division Duplexing (TDD) mode, the PSS and the SSS are placed in a different position within the frame.
Five-hundred and four (504) unique cell identities are supported in LTE. Specifically, LTE supports 168 cell ID groups, where each cell ID group includes three cell IDs. In other words, the 504 unique cell IDs are divided into 168 cell ID groups, with three cell IDs within each cell ID group. As a result, three PSSs (i.e., three PSS sequences) are used to provide the cell ID within a cell ID group. The two PSSs within a frame are identical, implying that the PSS is repeated every 5 ms in LTE. This enables the user terminal (i.e., the User Equipment (UE)) to determine the 5 ms timing of the cell and also correct the frequency offset. The position of the PSS gives the location of the SSS, which occupies the previous OFDM symbol. The sequences used in the SSS are interleaved in a different manner in the two subframes within a frame to obtain the correct frame timing and determine the particular cell ID group from the 168 possible alternatives.
The three PSSs are length-63 Zadoff-Chu sequences extended with five zeros at the edges and mapped to the center 73 subcarriers. The DC subcarrier is not transmitted, therefore, only 62 elements of the Zadoff-Chu sequences are used. The SSS is composed of two length-31 m-sequences interleaved in a specific manner in the two subframes (swapped in the frequency domain) to determine the correct frame timing. Since 31 cyclic shifts of an m-sequence are possible and all of them are orthogonal to one another, the SSS for a cell ID group is composed of two specific cyclic shifts of the two m-sequences.
The PSS and the SSS used for the conventional cell search procedure in LTE span the center 72 subcarriers. However, in NB LTE and similar NB systems, the bandwidth is, e.g., 200 kHz, which is about 12 subcarriers at the conventional subcarrier spacing in LTE of 15 kHz. As such, there is a need for a PSS and SSS structure for cell search in NB LTE or similar in-band NB system.